1. Field of the Invention
The present invention relates to a method of producing a multilayer piezoelectric resonator for use in, for example, a filter for a cellular phone.
2. Description of the Related Art
A length mode piezoelectric resonator which has a high design freedom in terms of characteristics, which has little spurious, and which makes it possible to increase a difference Δf between a resonant frequency and an antiresonant frequency has been provided (for example, described in Japanese Unexamined Patent Application Publication No. 10-4330).
FIG. 10 shows an example of a length mode piezoelectric resonator 30. The piezoelectric resonator 30 includes a substrate 31 having a plurality of piezoelectric layers 32 and a plurality of internal electrodes 33 alternately stacked upon each other, with the piezoelectric layers 32 at both sides of the corresponding internal electrodes 33 being polarized in opposite directions as indicated by arrows P. Insulating films 34 and 35 which cover every other internal electrode 33 are provided at opposing surfaces of the substrate 31. External electrodes 36 and 37 are disposed on top of the insulating films 34 and 35, respectively. Accordingly, the external electrodes 36 and 37 are alternately connected to every other internal electrode 33.
This type of piezoelectric resonator has a problem in that metals, such as Ag-Pd, used in the internal electrodes 33 diffuse in ceramic material during baking. When the metals diffuse, the effective area of each of the internal electrodes 33 is reduced, so that Δf is reduced.
To overcome this problem, the applicant proposed a piezoelectric resonator including float electrodes provided in its ceramic layers between the internal electrodes or in its outermost ceramic layers. The float electrodes are not connected to the external electrodes and are provided separately from the internal electrodes (Japanese Unexamined Patent Application Publication No. 2000-174581). In that case, the float electrodes restrict the diffusion of the internal electrodes in the ceramic material during baking, so that it is possible to prevent a reduction in the effective areas of the internal electrodes.
In a method of producing the above-described piezoelectric resonator, steps such as those described below are carried out. After a green sheet has been provided by forming a ceramic slurry into the form of a sheet, portions of this green sheet are punched out to have rectangular shapes to provide rectangular green sheets. An internal electrode is printed onto one of the green sheets using a first pattern, and is, then, dried. A float electrode is printed onto another green sheet using a second pattern which is different from the first pattern, and is, then, dried. Thereafter, the green sheet having the first pattern, a green sheet having a pattern which is inverted 180 degrees from the first pattern, and the green sheet having the second pattern are placed upon each other in a predetermined order, are aligned with reference to their external shapes, and are press-bonded together, so that a layered body is formed. The layered body is cut to form a unit, which is baked and polarized. After polarization, the unit is cut to form a strip in order to form insulating layers, external electrodes, etc., on the strip, after which the strip with the insulating layers and external electrodes disposed thereon is cut to form individual devices.
However, in order to prevent such float electrodes described above from coming into electrical conduction with the external electrodes, it is necessary to make the float electrodes smaller than the internal electrodes. Therefore, when printing the electrodes onto the green sheets, it is necessary to print the float electrodes using patterns which are different from the patterns of the internal electrodes. In addition, at least two types of green sheets having different patterns printed thereon must be provided. Consequently, there are disadvantages in that the production process becomes complicated and that costs are increased.
Further, in the related method of producing the above-described piezoelectric resonator, electrodes are printed onto the green sheets which have been previously punched out with rectangular shapes, and the green sheets are, while being aligned with reference to their external shapes, placed upon each other and press-bonded together. Therefore, positional displacements thereof during the printing directly affect the precision with which the electrodes are positioned. Consequently, the precision with which the electrodes are positioned is low, so that a desired electrical characteristic sometimes cannot be obtained.